Whilst HomePod works best with an Apple Music subscription,
allowing users to ask Siri to play any of the 40 million tracks in
the Apple Music catalog, it does not require a subscription to
function. We have learned that the HomePod can play content
purchased from iTunes, stream Beats 1, and listen to podcasts
without needing a subscription.

If you add music to your home iTunes library that was not acquired
through a purchase, HomePod will not be able to access it. It
appears HomePod doesn’t have Home Sharing, which would enable that
kind of feature.

Shouldn’t it work with iCloud Music Library? I get that it might not be able to access songs that only exist as MP3 files on your Mac, but if you have iCloud Music Library, it seems obvious that HomePod ought to be able to access them, no? It’s one thing if it doesn’t work with third-party streaming services like Spotify. But iCloud Music Library is Apple’s own thing.